A switched reluctance machine is a brushless, synchronous machine which has an unequal number of salient rotor and stator poles. There is a concentrated winding on each of the stator poles, but no windings or permanent magnets on the rotor. The stator pole windings on opposite stator poles are energized simultaneously. Ideally, the flux entering the rotor from one stator pole balances the flux leaving the rotor from the diametrically opposite stator pole, so that there is no mutual magnetic coupling among the phases.
In operation, there is an electromagnetic force of attraction between the rotor poles and stator poles of a switched reluctance machine. Hence, by properly positioning the stator phase excitation waveforms relative to the rotor angular position, forward or reverse operation and motoring or generating operation can be achieved. In particular, if the windings of two diametrically opposite stator poles are excited when a pair of rotor poles are approaching alignment with these excited stator poles, motoring torque is produced; and when the rotor poles are moving out of alignment with the excited stator poles, generating torque is produced.
A switched reluctance generator typically generates dc power. To generate ac power using a switched reluctance generator, an inverter is required. Disadvantageously, therefore, the generated power passes through two stages of power conversion, rather than one, resulting in a loss of efficiency.
A conventional switched reluctance generator system is a self-excited system that draws its excitation power from the same dc bus into which it generates power. Such a system is electromagnetically limited to a maximum load current that it is capable of delivering without losing its self-excitation. If the maximum load current for the particular switched reluctance generator system is exceeded, e.g., in case of a fault such as a short circuit, then the generated output voltage drops to zero. Unfortunately, since the system is self-excited, once the voltage drops to zero, generating operation cannot be resumed without external intervention; and a relatively large power source is required to supply the overload, clear the fault and re-excite the generator.
Commonly assigned U.S. patent application Ser. No. 07/968,642 of A. V. Radun et al., filed Oct. 29, 1992, which is a continuation-in-part of now abandoned U.S. patent application Ser. No. 07/859,754, now U.S. Pat. No. 5,289,107 describes a switched reluctance generator system having an excitation bus for providing excitation power thereto which is separate and distinct from a generator bus for providing the output dc voltage thereof. As a result, the average output current generated by the switched reluctance generator is substantially independent of the generator output voltage. Advantageously, therefore, the switched reluctance generator of U.S. patent application Ser. No. 07/968,642 is capable of automatically resuming normal generating operation following occurrence of a short-circuit fault, without requiring a relatively large external power source to supply the high short-circuit current, clear the fault and re-excite the generator.
In addition, U.S. patent application Ser. No. 08/101,623 of A. V. Radun, cited hereinabove, describes a switched reluctance generator system with self-excitation capability, even during load faults, without requiring multiple generator outputs.
Accordingly, it is desirable to provide a switched reluctance generator system capable of generating ac power, without requiring an additional stage of power conversion, that furthermore has self-excitation capability, even during load faults.